1. Field of the Invention
The present invention concerns a stepping motor driver for controlling rotational angular position and rotational speed of a rotor of a stepping motor.
2. Description of the Prior Art
With the high functionalization of systems equipped with motors, motors of which noise and vibration are low, and of which rotational speed ranges are wide, are being demanded. A stepping motor is caused to make a stepping rotation by changing instantaneously excitation currents for windings at each time when a set of external command pulses is given. Accordingly, it is necessary to reduce noise and vibration generated when the excitation currents are changed and to prevent the motor from stepping-out.
To reduce noise and vibration, and to prevent the motor from stepping-out, a micro-step drive using an inverter of a PWM (pulse width modulation) type to smoothly change excitation currents for windings is being generally adopted.
The micro-step drive is realized by feeding motor windings with stepped currents, obtained by transforming currents of sinusoidal wave forms, with phase differences according to the number of phases of the motor. Accordingly, it is necessary to control a plurality of phase currents according to the number of the phases of the motor. So, the construction of the driver becomes complicated with the increase of the number of phases of the motor.
To simplify the construction of the driver, it is conceived to apply an inverter controller that is generally used for controlling an AC servomotor, in which a coordinate transformation into a rotational coordinate system is adopted, to the stepping motor driver. The inverter controller technique is suitable for realizing a micro step drive with a high resolution, since the rotation angle can be controlled directly.
Examples of the application of the rotational coordinate control technique for stepping motors are still limited. Among these examples, there is a stepping motor driver for preventing the motor from stepping-out and for realizing a micro step drive, disclosed in Unexamined Japanese Patent Publication No.6-225595 (herein after referred to as the prior art). In this driver, phase currents in a fixed coordinate system are transformed into phase currents in a d-q rotational coordinate system, in which the d-axis is in the direction of the magnetic flux of the rotor and the q-axis is in the direction perpendicular to the d-axis, and the control of the currents applied to the motor are dealt with in the rotational coordinate system.
In the stepping motor driver disclosed in the prior art, assuming that the stepping motor is similar to a synchronous permanent magnet motor, angular position of the rotor is detected by an encoder connected to the stepping motor, and closed loop control systems for current control, for velocity control and for position control are composed. Detected signals representing the angular position of the rotor are transformed into signals in the d-q rotational coordinate system and the position control is conducted in the d-q rotational coordinate system. To simplify the construction of the control system, non-interference elements of the d-axis and the q-axis components are omitted, and the current commands are given directly on the d-axis and on the q-axis. In this driver, since the angular position of the rotor of the motor is controlled so as to make the detected position signal coincide with the command position, a micro step drive of the stepping motor in accordance with the resolution of the command position and that of the position detection device is possible.
In the stepping motor driver of the prior art as mentioned above, the direction of the current fed to the motor is made to coincide with the direction of the q-axis and the current is controlled according to the velocity deviation. Accordingly, a position detection device, a position controller and a velocity controller are required to be provided to effectuate the position control. Thus, there is a problem that the construction of the stepping motor driver is complicated, and consequently, is expensive.
Also in the stepping motor driver of the prior art, there is a problem that a vibration of the stepping motor occurs when it is stopped.
Further, in the stepping motor driver of the prior art, it is necessary to make the applied voltage to the motor always greater than, or equal to, the sum of the induced voltage and the internal voltage drop of the motor, in order to control the current in the q-axis direction according to the variation in the load. Since a stepping motor is a multipolar motor having some fifty pairs of magnetic poles in general, the voltage drop due to the inductance component is large. Accordingly, there is a problem that the controllable region of the stepping motor is limited and the stepping motor can not be controlled to a high rotational speed region.